The present invention relates generally to gas chromatography and particularly to controlling gas pressure in conjunction with gas chromatography. Gas chromatography (GC) entails the analytical separation of a vaporized or gas-phase sample. In a GC system, the sample is injected into a chromatographic column and is carried through the column by a chemically inert carrier gas such as hydrogen, helium or nitrogen. The carrier gas is utilized as the mobile phase for elution of the analyte sample in the column.
The carrier gas is typically introduced into the column at a location near where the sample is injected, e.g., at the head of the column, and thus carries the sample through the column. The column is typically housed in a thermally controlled oven. The column may be constructed of stainless steel, glass, fused silica, Teflon®, or the like. The column may be of the packed or open tubular (capillary) type. The column contains a stationary phase (particles, films or layers of a selected composition) by which different components of the sample are retained differently. Thus, as the sample flows through the column it becomes separated into discrete components of differing analytical (qualitative and/or quantitative) significance.
The eluent from the column flows to a detector provided with the GC system. Various types of detectors may be employed such as, for example, a flame ionization detector (FID), thermal conductivity detector (TCD), etc. The choice of detector often depends on the sample being analyzed. Moreover, the type of carrier gas utilized often depends on the type of detector utilized. Generally, the detector is of a type responsive to a property of the separated analytes (e.g., concentration) and converts the outputted flow of separated analytes to electrical measurement signals, which are then transmitted to a data processor. The data processor derives peak information or other useful analytical information from the measurement signals received.
A GC system typically utilizes a gas flow (flow rate and/or pressure) regulator to control (switch on and off) the flow of carrier gas to the GC column. For the GC system to operate properly, the carrier gas must flow through the column at a particular working pressure (i.e., column head pressure). Conventionally, the gas flow regulator attempts to increase (switch on) or decrease (switch off) the pressure to the working (set-point) pressure as fast as possible. Moreover, the carrier gas may be provided by a carrier gas supply source that is initially pressurized at a pressure much different from the set-point pressure, and/or the GC column may initially be at ambient pressure which may be substantially different from the set-point pressure.
Thus, due to the rapid changing of the gas pressure, the compressibility and expansion of the carrier gas, and the fact that the working pressure may differ substantially from the ambient pressure, the conventional operation of the gas flow regulator may cause pressure pulses in the column. Such pressure pulses may cause particles of the stationary phase contained in the column to become loose and flow through the column. The loose particles may accumulate and block flow through the column, become lost, and generally render the column unusable.
This is particularly true in the case of Porous Layer Open Tubular (PLOT) capillary columns. A PLOT column is typically constructed of fused silica or steel tubing. The inner wall of the tubing is coated with different porous adsorbents held on the inner wall mainly by London/Van der Waals forces. Particles from the porous layer may become loosened by pressure pulses.
In view of the foregoing, there is a need for controlling the pressure of a carrier gas flowing into a GC column in a manner that avoids or at least substantially reduces pressure pulses.